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Around the globe each year, ten trillion gallons of storm water pollution—a combination of rainwater, oil grease, heavy metals, pesticides and raw sewage—wash off city streets into lakes, rivers and oceans. It is the fastest growing source of water pollution worldwide.

Many U.S. cities never planned to deal with the volume of storm water they now face. The systems, in some cases designed in the 1800s, have reached or exceeded their useful life, and often rely on antiquated technology, which combines raw sewage and storm water—commonly referred to as combined sewers—that overflow directly into rivers and streams during large rainstorms. Updating these systems (e.g. sewer separation or underground storage) is often cost prohibitive or simply not technically feasible. It is not uncommon to find pipes spewing sewage into water adjacent to some of the most expensive real estate in the world, such as the west side of Manhattan or in the Georgetown neighborhood of Washington, DC.

And even if a city completely eliminates its combined sewer overflows, separated sewer systems send untreated, polluted storm water directly into rivers and streams causing habitat destruction and stream bank erosion, which can lead to flooding, among other issues.

Urban growth brings more concrete, brick, cement and other impervious surfaces that generate storm water run-off. Further, storm water run-off is a Federally-regulated pollutant under the U.S. Clean Water Act, requiring many cities—often through Federal consent decrees—to upgrade systems in order to come into compliance. Combine more run-off with systems in need of repair and municipal budget shortfalls, and one can imagine how challenging it is to slow or stop this massive pollution source.